1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus used in photolithographic processes for manufacturing of semiconductor device elements, image pickup elements (CCD), liquid crystal display elements, plasma display elements and thin film magnetic heads. The method is particularly suitable for controlling light exposure of an exposure beam from a pulse light source used in scanning type projection exposure apparatus based on the step-and-scan method. This invention is based on a Japanese Patent Application, First Publication, Hei 11-326192, the content of which is incorporated herein by reference.
2. Description of the Related Art
One of the basic functions of the projection exposure apparatus for manufacturing of semiconductor device elements, for example, is to control the integrated exposure level on the object to be exposed within a suitable range, at each point in each shot region of a wafer (or glass plate) that has been coated with a photoresist coating (photo-sensitive material). Conventionally, regardless of whether a continuous light source such as mercury lamp or a pulsed light source such as excimer laser light source is used to expose the wafer, exposure control for a static exposure type apparatus such as the conventional stepper is based on the so-called xe2x80x9ccutoff controlxe2x80x9d, in which the exposure is continued until the integrated exposure on the wafer measured indirectly by the detector, comprised by an integrator sensor disposed in the illumination optical system, exceeds a specific exposure value (target exposure level) corresponding to a critical level.
When the exposure light source emits pulses of laser light, because individual pulses have different levels of light energy, it has been a practice to assure reproducibility of precision in exposure control by applying more than a certain minimum number of exposure pulses. In such a case, because the minimum exposure energy level is low for a high sensitivity photoresist material, it is necessary to place a light reducing member in the optical path to reduce the power of the laser pulses uniformly so as to assure delivery of pulses exceeding the minimum number of exposure pulses.
Further, for scanning exposure type apparatus based on the step-and-scan method, which has been in use in recent years, a conventional method (open-level exposure control method) is used, in which the exposure light (exposure beam) containing pulses of laser light is simply accumulated by integrating the light energy. In such a method, it is necessary to adjust the pulse energy so that a linear control can be applied to obtain a desired degree of exposure control as computed from the following relation. In other words, the pulses must be counted in whole numbers.
(target exposure level)=(number of pulses)xc3x97(average energy per pulse)
where a value for the average energy per pulse is to be obtained by the integrator sensor immediately prior to an exposure event.
In contrast, as disclosed in a recent Japanese Patent Application, First Publication, Hei 6-252022 and in a corresponding U.S. Pat. No. 5,627,627, the pulse energy of individual pulses is controlled by determining the values of individual pulse energy in real-time during the exposure process so that an integrated energy level of prior pulses can be used to determine a target value for the pulse energy of a next group of pulses. This per-pulse exposure level control method enables to minimize the scatter in the integrated exposure levels compared with the open-level exposure control method.
As outlined above, although there have been proposals for various types of exposure level control method, when it is necessary to change the exposure level over a relatively wide range of transmittance, all of these methods require the use of a specific light reducing member to lower the power of the exposure light (either pulsed or continuous). In such a process, it is necessary to mechanically switch the optical filters having different light transmittance characteristics in the light reducing member, and, immediately after changing the transmittance, to perform test emission of light source to measure the power (exposure energy) of the exposure light transmitted through the filter, and to re-adjust the exposure conditions to be consistent with the measured values of the existing power (exposure pulses per one point on the wafer if the exposure light consists of pulses).
It should be noted that, deviations in the line widths of the circuit patterns formed on the wafer are caused by variations in the thickness of the coating applied on the wafer in the course of applying the photoresist coating, and by a related phenomenon of non-uniformity of standing waves within the photoresist coating, as well as non-uniformity in developing the patterns. Such errors in line widths cause errors in the line widths of the circuit patterns on each layer of integrated circuit fabricated on the wafer. Therefore, as the density of circuit integration of semiconductor devices increases further, there is a danger of lower yield of final product caused by such errors in the line width. To correct such line widths errors in the photoresist pattern, it is necessary to conduct a series of test exposures. For example, several evaluation wafers are prepared by applying a photoresist coating and performing test printing by varying the integrated exposure level over the coated wafer by a given amount. After developing the photoresist pattern, line widths of resist patterns in each shot region are measured so that the exposure level that produced a line width nearest to the design value can be chosen as the correct exposure level for that shot region. The distribution of target exposure level thus obtained is roughly concentric about the center of the wafer, for example, so that it is considered practical to divide the entire shot region into a number of sub-regions and to determine a proper exposure level for a group of sub-regions. Also, it may be considered that the variation in the target exposure level in various sub-regions is about xc2x110% with respect to the average value of the exposure level.
Therefore, when different exposure levels are assigned to a plurality of sub-regions in the wafer, use of the conventional light reducing member leads to the necessity of performing exposure testing whenever the exposure power is changed in the course of successive exposures of various shot regions over the wafer. Such a procedure leads to increasing the time necessary to process each wafer and lowering the throughput of photolithographic process.
It is therefore a first object of the present invention to provide an exposure method and an exposure apparatus to enable to prevent the reduction of the throughput, without decreasing the control of exposure level precision, when exposing a plurality of regions (or sub-regions) defined on a wafer at different target exposure levels.
It is a second object of the present invention to provide an exposure method and an exposure apparatus based on the scanning exposure method using pulses of laser light to prevent the reduction of the throughput and the loss of precision in exposure level control when exposing sub-regions on a wafer at different target exposure levels.
It is a third object of the present invention to provide an exposure method and an exposure apparatus to enable to quickly determine target exposure levels in a plurality of regions defined on a substrate base such as a wafer.
It is a further object of the present invention to provide a manufacturing method for high precision devices based on the present exposure method.
A first method is for exposing a pattern formed on a first object (11) onto a sub-divided region defined on a second object (14), and exposing the pattern successively on a plurality of divided regions (31(2, 1))xcx9c(31(5, 6)) defined on the second object so as to replicate the pattern in each of the divided regions, by moving the first object synchronously in relation to the second object through pulses of exposure beam emitted from a pulsed energy source (1), wherein the plurality of divided regions includes a plurality of subdivided regions having different target exposure levels, the method comprises the steps of: selecting a transmittance of light reducing member (3) disposed in an optical path of the exposure beam based on at least one target exposure level of the plurality of target exposure levels; and, when exposing the divided regions having different target exposure levels, adjusting exposure level control parameters according to individual target exposure levels without changing the selected transmittance of the light reducing member.
The exposure level control parameters, for example, include at least one parameter from a group of parameters that includes a width in the moving direction of the second object disposed in the path of the exposure beam, a moving speed of the second object, an oscillation frequency of the exposure beam produced by the pulsed energy source, and an energy of the exposure beam emitted from the pulsed energy source.
According to the present invention, after finishing scanning exposure of the first divided region and before performing scanning exposure of the second divided region, a new target exposure level can be obtained by adjusting the parameters that can control the integrated exposure level (exposure level control parameters) without changing the setting of transmittance of the light reducing member. Accordingly, it is possible to eliminate those processes required for test emission, mechanical adjustments (switching of filters in the light reducing member, for example), thereby preventing the reduction of the throughput of the projection exposure apparatus. In the present method, because the pulses of laser light are used, it is preferable, within a range of appropriate frequencies, to also satisfy a condition that each divided region on the second object be exposed with a number pulses exceeding the minimum number of pulses.
Also, in the present invention, target exposure levels are assigned to individual divided regions according to distances from a center of the second object, for example. This method is useful in adjusting the exposure levels when the differences in the line widths (produced after photographically developing the exposed pattern) are distributed roughly concentrically.
A second method for determining an exposure level of an exposure beam that illuminates a first object (11) and exposes successively a plurality of divided regions defined on a second object (14) so as to replicate an image of the first object in each of the divided regions, is to pre-determine different levels of target exposure levels for the plurality of divided regions defined on the second object, so that, when successively exposing the divided regions (31(2, 1)xcx9c31(5, 6)) defined on the second object, exposure levels of the exposure beam required for each divided region are assigned by changing exposure parameters without mechanically switching optical components or performing test emissions of the exposure beam.
The present invention prevents the reduction of the throughput as in the first exposure method when changing the exposure level during the exposure process, because mechanical switching of filters and test emission of exposure beam are not necessary.
In the above exposure method, the exposure beam is comprised by pulses of laser beam output from a pulsed light source, and when each divided region on the second object is exposed by moving the second object relative to the first object through the exposure beam in synchronization, it is desirable that at least one of the control parameters including an oscillation frequency of the pulsed light source, a target pulse energy of each pulse emitted from the pulsed light source, and a scanning speed of the second object, is changed in order to assign an exposure level of the exposure beam to each divided region. This is an application of the second exposure method to the scanning exposure process, so the above condition to expose with a number of pulses exceeding the minimum number of pulses can be satisfied by changing one of the three exposure parameters or one combination of the exposure parameters.
Next, a first exposure apparatus for emitting an exposure beam to illuminate a first object (11) and exposing a plurality of divided regions defined on a second object (14) successively with an exposure beam through a pattern formed on the first object, is comprised by: a pulsed light source (l) for generating pulses of light to serve as an exposure beam; a stage system (15, 19, 20) for moving the first object synchronously with the second object; a memory section for storing target exposure levels in a plurality of different levels for a plurality of divided regions defined on the second object; and a control system (26) for adjusting at least one exposure parameter in preparation for successively exposing the plurality of divided regions defined on the second object according to the target exposure levels stored in the memory section (26a), wherein the one exposure parameter includes emission frequency of the pulsed light source, target per-pulse energy of the pulses emitted from the pulsed light source, and speed of scanning the second object controlled by the stage system.
A third exposure method is for determining an integrated exposure level in a process of successively projecting an image of a first object (11) through a projection optical system (13) on a plurality of divided regions defined on a second body (14), by detecting a level of reflected light reflecting from the second object or an evaluation body (14P) in place of the second object through the projection optical system; and determining a target exposure level for each of the plurality of divided regions defined on the second object.
According to this exposure method, the distribution of the target integrated exposure levels (set light exposure level) to produce optimal line widths, produced after developing a photo-sensitive material such as photoresist applied on the second object, is sometimes governed by the film thickness. In such cases, by utilizing the fact that reflected light level is altered by the standing wave effects that is dependent on the film thickness, reflection light level from individual regions in the plurality of divided regions on the second object is measured, and film thickness in respective regions is determined according to the results of measurement. Then, target integrated exposure level can be optimized in each divided region according to pre-determined relationship of the film thickness to the optimal exposure level.
A second apparatus for determining an integrated exposure level in a process of successively projecting an image of a first object (11) through a projection optical system (13) on a plurality of divided regions defined on a second body (14), is comprised by: a detector (60) for detecting a level of reflected light reflecting from the second object through the projection optical system; and a control system for determining a target integrated exposure level for each of the plurality of divided regions according to output data from the detector.
In this case, it is preferable that the detector is used also for adjusting focusing properties of the projection optical system. According to this arrangement, the detector can determine the light level of the light reflecting from the second object and propagating through the projection optical system, so that changes in the aberration characteristics and other changes in the system can be determined from the measured results. Therefore, it is possible adjust the focusing properties of the projection optical system according to such real-time changes in the operating characteristics of the system.
A method of manufacturing a device according to the present invention includes the steps of imprinting a device pattern on a workpiece using the methods or the exposure apparatus described above. According to this method, the pattern formed on the first object is a device pattern and the second object is a workpiece (a substrate such a wafer), and because the integrated exposure level can be controlled with precision, high precision devices having superior performance properties, such as controlled widths of fine circuit lines, can be mass produced.